1. Field of the Invention
The present invention relates generally to satellite attitude sensor bias estimation and, more particularly, to a method and apparatus for estimating and nulling attitude sensor bias.
2. Description of Related Art
The term attitude is used to describe the orientation of an object with respect to a reference orientation. Attitude is of particular interest in satellite operations. For example, if a satellite is to be used in a communications application, it is necessary that the satellite be oriented in the proper direction to receive and/or transmit relevant information for the communication link. The attitude of a satellite is determined by computations based on the output of attitude sensors located on the satellite. Attitude sensors monitor the position of reference objects, which may include the sun, the earth, constellations, or beacons. For example, a spacecraft may have two sensors, one monitoring the sun and one monitoring the earth. It is well known that given position information from two reference objects (e.g., the sun, the earth, stars, or beacons), it is possible to determine spacecraft attitude.
Due to external and internal factors, the attitude sensor outputs may become biased. For example, thermal expansion and contraction of the spacecraft may move the sensors, thereby biasing the sensors by moving the reference frame of the spacecraft from the intended position. Attitude sensors may also be susceptible to electrical noise. Movement of sensors and electrical noise can lead to errors in the calculation of the spacecraft attitude.
Known methods of keeping bias and noise to a minimum typically involve extensive attitude sensor calibration and tight thermal control of the sensor mounting structure. These methods are tedious, time consuming, and expensive. Therefore, it is desirable to have a method and apparatus for removing attitude sensor bias and noise that eliminates the need for time consuming and expensive sensor calibration and thermal control of sensor mounting.